This invention is concerned with filters constituted of paper-like or other filter sheet material that is folded to a zig-zag shape, for instance so-called absolute filters, that is filters where practically no impurities pass through the filter. To obtain the least flow resistance through the filter, it is desired that the filter is zig-zag shaped with wedge-like cross-sections on each side of the filter. In this way, the flow through the filter is smooth and continuous with the same flow velocity all the way through and since the pressure over the filter surface is even, the entire surface is utilized. To ensure that the filter functions properly it is necessary that the wedge-like structure be kept open. This is, for instance, done by the insertion of wedge-like distance elements made of corrugated aluminum strips. The fabrication of these wedge-like strips is however rather expensive due to the tooling costs and of course also the cost of the aluminum material itself. Another way of securing the wedge-like shape is of course to hold the filter material in a surrounding fixture or frame. This is also costly and furthermore unsuited for large filters.
In view of the above comments, it is an object of the invention to obtain a filter with a wedge-like cross-section in a simple manner that is easy to carry out and therefore also possible to fabricate at low costs.